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Engineering Edge

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Sometimes even the most deeply considered engineering creations such as architectural structures can exhibit unpredictable behaviors. One such example, is the commercial skyscraper in the center of London, 20 Fenchurch Street. Nicknamed The Walkie-Talkie Building or The Pint because of its distinctive top-heavy shape, this impressive building provides 680,000ft² of exclusive office space that offers unrivalled panoramic views of London. The only problem is that the £200million skyscraper has a dazzling effect on passers-by. As a result of its unusual shape, the architectural structure reflects blinding rays of sun onto the street below, damaging the vehicles parked beneath it.

Figure 1 20 Fenchurch Street Skyscraper and corresponding CAD-model

In the summer of 2013, the United Kingdom experienced unusually hot and sunny weather. On the 29th of August, Martin Lindsay, director of a tiling company, parked his Jaguar XJ for one hour opposite The Walkie-Talkie Building only to return to find a rancid smell of burning plastic and that parts of the car, including the wing mirror and side panels had melted or warped. We later calculated that Mr Lindsay had, by coincidence, parked in the 30m area with the most intensive sunlight exposure from Walkie-Talkie. This article investigates this phenomenon with the aid of Mentor Graphics’ FloEFD analysis software.

To begin a full-scale CAD-model of building and the surrounding landscape was reconstructed with the maximum correspondence to its parabolic surface (Figure 1). The area topology data is taken from Google maps and the solar radiation parameters (location and actual time) were defined in FloEFD automatically.

As a first step in the investigation we estimated the ray exposed area dynamics as a result of the sun’s position in 29th of August. Distribution of the calculated solar net radiation flux, W/m2 (indicated here as Insolation), for the expanded time interval from 10:10 to 15:10 is shown in Figure 2. We can see the complicated shadow distribution changing its configuration as the rays shift. The sunlight focus spot illustrating via insolation maximum moves from west to east heating the pavement and any objects on it.

Figure 2 Solar net radiation flux (insolation) as a function of time. Hourly simulation from 10:10 to 15:10, 29th Aug, 2013

By analyzing the results obtained, one can see that the most heated area at the time of incident (between 12:00 and 14:00) is the section of Eastcheap between St. Mary-in-Hill Street and Botolph Lane (Figure 3). The next step in the calculation, was to place the sunlight focus area in the CAD model. (Figure 3)

Figure 3 Spread of probable Jaguar positions between of 12.00 and 14.00

A more exact car position was defined by the focus trajectory analysis. For a deeper understanding of the optics specific to this case, we compared the results of the solar radiation flux calculation both with and without reflection. In the first case, the parabolic surface of the skyscraper was defined as reflecting glass and at the second case as absorbent concrete. Figure 4 illustrates that the parabolic reflecting surface is a necessary condition to focus the sunlight.

Figure 4 Comparison of solar radiation influence with or without focusing effect at 12:40.

In order to simulate the melting effect of the wing mirrors, their materials were defined as plastic. For the car’s shell material, steel was used. Figure 5 illustrates the solar radiation influence on the car parked on the southern side of Eastcheap near the crossing with Botolph Lane at 12:40. The focused insolation maximum reaches the car’s bonnet (hood) and left wing mirror shell at this point.

Its value is around 1300 W/m2 which is about 1.5 times higher than the average solar radiation flux value (about 850 W/m2) relative to current location, day and time. According to the calculations, the focused insolation maximum is reached within the morning hours of the time period of 10:00 and 15:10. It is an incredible feat of luck for pedestrians that the focused area (about 2600 W/m2!) is occupied by buildings which is preventing more dramatic damage.

The temperature distribution shows the hottest area on the left wing mirror shell. The solar influence on the wing mirror surface lasted just ten minutes. This short time is enough to cause a considerable heating effect. Typically, a wing mirror has a hollow construction with thin plastic outer shell. Given these variables it is rather difficult to remove the heat inside the construction and therefore the plastic shell is forcibly heated. The softening temperature of the typical plastic materials used for wing mirror shells is around 100ºC, with the melting temperature at around 220ºC. The result in the simulation model exactly mimics the effect on Mr Lindsay’s Jaguar: the most heated parts of wing mirror are hot enough to change shape or even melt. (Figure 5).

The measurement of the pavement temperature at the focused rays area was reported by local press as being around 59.5ºC. This fact was obtained in FloEFD calculations (see Figure 6).

Figure 5 Insolation of all model surfaces and the temperature distribution on the car.

The retrospective of the focus position and intensity was obtained under the assumption that the skyscraper front surface is made of ordinary glass. Its reflective properties are dependent on the incidence angle and other factors. Glass reflection forms a complex interaction pattern which is defined by ray optics principles. Perhaps this consideration was not completely taken into account in the design of the skyscraper. If even the ordinary parabolic glass surface working as a giant mirror can scorch plastics, then how dangerous could it be with more reflecting materials?

Figure 6 The pavement temperature distribution.

Let us consider the extreme situation where the solar radiation flux value in focus reaches its physically feasible maximum. Obviously this is a case of total or mirror reflection. The FloEFD simulation of total reflection shows that maximum of insolation at 12:40 is about 6000 W/m2 (Figure 7). No doubt the consequences of such exposure would be more dramatic.

Figure 7 The pavement temperature distribution.

Conclusions

This example effectively illustrates designers and architects should not neglect the CFD engineering analysis when designing complex structures. While designing an embedded item the interference of all components becomes a very important factor that can have an influence in all assembly operations. Using FloEFD, it is quite simple to explore various constructions under different conditions in details. The reliable results of calculations are based on a consideration of geometrical optics as well as heat transfer specific data.

The amount of engineering reference data that is available in FloEFD makes the designer’s task easier. For example in this task the parameters of solar radiation as well as the material’s physical properties were defined by means of FloEFD’s database. The use of FloEFD in the design of new structures is an effective way to prevent costly failures in the future. Read the article online: bbc.co.uk/news/uk-23945767

Reference

  1. maps.google.com
 
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